That's one option.
The EU said that an extension until May 22 is available on the condition that the WA is agreed prior to April 12. That has been agreed and cannot be cancelled. This would be only to allow UK to put in place proper legislation as an ex-EU member state.
The EU have a summit meeting diaried for April 10. (yes! six days time, not including weekend!) If no agreement on WA is achieved by then, and TM wishes to avoid a No Deal crash out, TM must attend and request a further extension. There is no automatic right to expect this. It only takes one EU member state to not agree and the request is not granted, we crash out two days later on April 12, unless Article 50 is revoked before that.
Or UK can decide to crash out anyway, but TM appears to have ruled that out now, although it is still the default position and she has refused to change that default position.
EU have specified April 12 as the latest date for UK to decide because immediately after that, arrangements begin for EU Parliament elections. UK must have decided whether they are in or out of those elections. (UK seats are redistributed if they are not in)
TM is hoping to ask for a 'flex-extension, up until May 22, where she can pull the plug on EU elections prior to then if an agreement can be achieved in UK Parliament. I don't see how that can be granted, obtained or achieved.
After April 12, EU and all member states start gearing up for EU elections. If no-one is sure if UK are in or out, then no-one can know which seats are available. Also, UK must start gearing up for those elections, in case they have to participate. She is intending to ask for this flex-extension as a compromise to her Brexiteer cabinet ministers, who otherwise would resign. But I cannot see EU (the individual member states) agreeing to it
Alternatively, TM can ask for a longer extension (say until end of 2019, or even longer) but must present a credible plan and reason for that extension. Of course, on that basis UK must participate in EU elections. Something which is anathema to committed Brexiteers.
TM does have executive authority, in this summit meeting to decide for herself.
